One more request, I want to bold only the Chapter titles, but not the sections and subsections in TABLE OF CONTENTS. I want remaining sections and subsections to be in normal series and NOT bold. Please, check the attachment and help me with code.

I haven't addressed any of your questions so far. The problem is your class file, which is unknown. There used to be a template defining a class called Thesis.cls and it was terrible. There have been serious copyright and license issues and basically everybody using the class broke some law of some country. Luckily/Unfortunately, nobody cares about that.

For a minimal document, chapters are in bold in the table of contents. So, something in your document is changing that. Maybe the class file, maybe something in your very very long preamble. I don't know, i didn't test.

Over many years, there appeared several different classes thesis.cls. That's because on different universities people developed kind of templates and classes for the students theses, and called it the same way, that's a bit careless. So we cannot know which thesis class or template you are using.

I usually based on the assumption, that it's like a report class plus some tweaking. That could be a good guess. Where did you get your thesis.cls or thesis template from? Do you have a link, where we can download it and test it?